EP3338936B1

EP3338936B1 - Method of joining elements to components

Info

Publication number: EP3338936B1
Application number: EP17178341.8A
Authority: EP
Inventors: Christian Reis; Gerson Meschut; Bah EISSARA
Original assignee: Newfrey LLC
Current assignee: Newfrey LLC
Priority date: 2016-12-23
Filing date: 2017-06-28
Publication date: 2021-12-08
Anticipated expiration: 2037-06-28
Also published as: JP2020501912A; JP7025431B2; WO2018114293A1; US20230125406A1; US20190344374A1; EP3988234A1; EP3988234B1; EP4279557A2; DE102016125599A1; EP3338936A1; EP4279557A3; US11541476B2

Description

The present invention relates to a method of joining joining elements to components.
Joining method of the above-mentioned kind and joining devices are widely known, especially in the field of so-called stud welding or stud gluing.
In these methods, joining elements such as studs are joined to components such as plates in such a way that the studs protrude perpendicular to a surface of the component. Such joined arrangements can be used to attach clips made from plastics material, for example, to the stud. The clips may, for example, be used to fix pipes or cables in relation to the component, such as, for example, fuel pipes, brake pipes or electrical cables. The generic joining method is therefore used in particular in the field of bodywork manufacturing for motor vehicles.
In stud welding, an electrical current flow is established between the joining element and the component, the joining element being raised above the component so that an arc is generated between said components. The arc causes the opposite joining surfaces of the component and the joining element to melt. The joining element is then lowered onto the component so that the electrical joining current is short-circuited. The entire molten mass solidifies and the joining process is complete.
In stud gluing, an adhesive which can be activitate is generally applied to one joining surface of a joining element beforehand. Stud gluing then takes place by activating the adhesive. The joining element and the component are then pressed against one another and finally the adhesive is cured. This can be achieved by a variety of factors, such as by applying heat, for example.
The joining process itself is not the only factor responsible for the quality of such joints. The material properties and the surface quality of the component, and also the joining element in some cases, also play a not insignificant role in this process. This applies if the component and the joining element are manufactured from a steel material. Besides, this problem applies if the component and the joining element are each manufactured from an aluminum alloy.
Changes in the characteristic properties of the component are particularly noticeable in joints based on aluminum alloys. Such properties may include whether the aluminum alloy is a recycled material. In addition, there may also be problems with regard to irregular grain sizes on the upper layer, which may be up to 1 mm deep, and in particular when using extruded material.
Irregular grain sizes may lead to different conductivity values. As a result, this may affect the current flow through the arc.
Many components are also manufactured using casting processes. In such cases, the surface is coated with release agents, which may include waxes, oils, polysiloxanes, hydrocarbons, polymers, etc. If the coating or the coat comprising such release agents is unevenly distributed over the surface, it is particularly difficult to adapt the joining parameters appropriately. If coated with carbon, this can lead to pores or cavities in a welded joint, or in other words to a higher porosity of the welded joint overall, which may have a detrimental effect on its strength.
In addition, alloy components may also have an effect on weldability.
As a general rule, components with defined surface specifications are required, but practice suggests that these surface specifications, to which a joining process is then specifically adapted in relation to joining parameters, are not always observed satisfactorily.
In stud welding, the use of an arc cleaning ("clean flash") process before the actual stud welding process is already known in the art. In this case, an arc is created between the joining element and component with alternating polarity before the welding process, causing impurities to be ionized and detached from the component surface. The problem with this process is that such impurities may accumulate on the joining surface on the stud, as a result of which problems may still arise, even in this case, with regard to the consistency of the joints. WO0069593 discloses a two-stage arc welding process, wherein a generated plasma cleanse the weld-on region. US3835284 (basis for the preamble of claim 1) discloses a surface cleaning step using a welding gun discharging while it holds a tungsten electrode. US2016/064195 discloses a plasma treatment of an elastomeric material, such as a shoe sole, for adhesion. The plasma is applied to increase carbonyl functional group within an altered region of the elastomeric material. US5587093 relates to an arc head comprising means for introducing a gas flow ionizable to produce a plasma. The arc head can be configured to produce an accelerated plasma plume for surface cleaning of weld samples prior to coupling an arc column to the weld sample for welding.
In the light of the above, one object of the invention is to provide an improved method for joining a joining element to a component.
The above object is achieved on the one hand by a method of joining joining elements to components as defined in claim 1.
The cleaning methods according to the invention each differ from the clean flash cleaning method mentioned initially, in which an arc with alternating polarity is created between the joining surfaces using stud welding equipment, which at least causes the joining surface of the component to be cleaned.
The cleaning methods according to the invention, which differ from such a clean flash method, allow only one of the joining surfaces or both joining surfaces, one after the other, to be cleaned in a targeted manner.
The cleaning step preferably entails cleaning the joining surface using a physical cleaning medium, which differs from an arc created between the joining element and the component.
In other words, the cleaning step entails a cleaning process which is carried out independently of the joining process.
The cleaning step is preferably carried out in one stage, in which a joining element is already in a retaining device of a joining head and is assigned to a specific position (joining position) on the component. This is particularly advantageous if a cleaning device for carrying out the cleaning process is arranged on the joining head.
Alternatively, it is possible to carry out consecutive cleaning processes at each of these joining positions on a component on which a plurality of joining elements, for example, are to be fixed, in some cases even before a joining element is placed in a retaining device of a joining head if applicable. The cleaning process or cleaning processes may thus be performed on one component or jointly, so that all joining elements can then be attached to the component, no further cleaning process being required between the joining processes.
As mentioned above, the cleaning process is preferably carried out using a cleaning medium.
The cleaning medium is according to the present invention a gas. The cleaning medium is preferably applied to the component by means of a separate cleaning device, which directs the cleaning medium onto a joining surface, particularly onto a joining position on the component. The cleaning device used to apply the cleaning medium is for example designed such that it is separate and independent from the technology used to carry out the joining process.
The joining method and the joining device according to the present invention can preferably be combined by a step in which at least one characteristic variable of the component and/or the joining element is recorded and subsequently evaluated. The evaluation may, in some cases, allow for the fact that it is necessary to carry out a preliminary cleaning process according to the invention if a joining position is evaluated negatively. In other cases, in which a joining position receives a good preliminary evaluation, it may not be necessary to carry out such a cleaning process before carrying out the joining process.
The characteristic variable in this case may relate to the material, the surface quality, surface processing, carbon coating on the surface, cleanliness, and may relate to release agents in the case of a cast workpiece, but may also include relative variables such as the component material in relation to the joining element material, for example.
According to an embodiment, only at least one characteristic variable of the component is recorded and only one joining surface of the component is cleaned, if this is necessary. All subsequent references to recording and evaluating a variable of a component and to cleaning a joining surface of the component should, however, relate equally to recording or evaluating a variable of the joining element and cleaning a joining surface of the joining element unless otherwise explicitly specified.
Such a characteristic variable is preferably recorded automatically and specifically preferably by means of an appropriate recording device. This recording device or these recording devices may include suitable sensors which either work on a purely passive basis or where the workpiece actively undergoes a physical process, in which the subsequent reaction to this process is recorded by sensors.
Such an active recording process may, for example, entail an electrical conductivity measurement using an eddy current measurement method, or even a surface coating measurement using fluorescence excitation, or contact resistance measurement.
In the case of fluorescence measurement, light in the visible range or in the UV range may be applied to the joining surface, and the resulting excited fluorescence radiation (usually in a different frequency range) is then recorded. Individual photons can be "counted" in particular, the number of recorded photons or light quanta usually being in correlation with the thickness or density of a coating on the joining surface of the component.
In the case of electrical conductivity measurements, an alternating magnetic field may, for example, be induced in the component surface. As the component is preferably a non-magnetic material such as an aluminum alloy, this gives rise to eddy currents in the component, which in turn generate a magnetic field. This reaction field can then be recorded. The magnitude and intensity of the reaction field may be an indicator of specific material properties, such as hardness, thermal conductivity, homogeneity or similar properties of such a component. In particular, the reaction field correlates to electrical conductivity.
In the case of contact resistance measurements, a contact is placed on the joining surface and a voltage between the contact and the component is increased and/or a force by means of which the tip of the contact is pressed onto the component is increased. The thickness and/or density of a coating on the surface can be deduced as a result of the alternating electrical resistance resulting from this action.
The object of the invention is thus achieved in its entirety.
According to the invention, the plasma gas cleaning method entails generating a non-transmitted (or non-transferable) arc between a tungsten electrode and an anode surrounding the tungsten electrode, said arc generating plasma when using a plasma gas, said plasma being directed onto the joining surface. In this case a tungsten electrode is understood to mean an electrode manufactured from a metal with a very high melting point, or in other words, in particular an electrode made from a material such as tungsten, which does not melt when an arc is generated.
The arc is generated between the tungsten electrode and an anode made from an electrically conductive material surrounding the tungsten electrode. In other words, an electric arc is not generated between the tungsten electrode and the component or its joining surface in this step. As a result, this is a non-transmitted (or non-transferable) arc. The plasma or the "plasma arc" cannot be electrically conductive as a result of this measure and can therefore preferably not be deflected by magnetic means. Accordingly, the plasma arc can be focused satisfactorily and is preferably not deflected or only slightly deflected from a cleaning axis (joining axis) as a result.
The plasma or the plasma arc which is directed onto the joining surface causes any surface coatings on the joining surface to evaporate without these materials subsequently accumulating on the joining surface of the joining element.
Standard impurities such as oil films, grease, etc. can be removed particularly well.
In this process it is particularly preferable if the plasma gas is passed under pressure into an intermediate space between the tungsten electrode and the anode, the plasma being discharged from the intermediate space towards the joining surface.
The gas pressure also ensures that a coating of this kind on the joining surface is also eliminated from the surface as a result of the gas pressure, or in other words, an oil film can be driven outwards in the form of a ring.
According to another preferred embodiment, the anode is connected to a plasma gas nozzle at an end located downstream in the direction of the plasma gas discharge direction, said nozzle combining the plasma or plasma arc emerging from the intermediate space.
A very narrow plasma arc can be produced as a result, said arc preferably comprising a conical shape when it emerges from the plasma gas nozzle with a cone angle of < 15°, particularly < 10°.
The plasma emerging onto the joining surface, which is also referred to as the plasma arc, is therefore very directionally stable. Position deviations between a programmed position and an arc deflection due to blowing effects are therefore very minor. The plasma arc may also be stable if the distance between the plasma gas nozzle and the component fluctuates. Such a plasma arc can also continue to function in a stable manner even with low electric currents.
An inert gas or similar is preferably not generated around the plasma arc, as the joining surface is not melted by means of the plasma arc in the region of the joining surface, or in other words, the presence of oxygen or similar at the cleaning region is not generally a problem.
According to a preferred embodiment, a distance ranging from 2 mm to 25 mm is adjusted between the plasma gas nozzle and the joining surface during the cleaning step. The distance preferably ranges from 2 mm to 50 mm, or particularly from 3 mm to 10 mm.
It is also advantageous if the ratio between a nozzle diameter of the plasma gas nozzle and a distance adjusted between the plasma gas nozzle and the joining surface during the cleaning step ranges from 1:4 to 1:1.
In this case the nozzle diameter of the plasma gas nozzle is preferably the internal diameter of the plasma gas nozzle, or in other words the effective diameter through which the plasma emerges from the plasma gas nozzle.
In particular, this ratio may range from 1:3 to 1:1.5.
It is also advantageous if the anode and/or a plasma gas nozzle connected to the anode is cooled by means of a cooling device.
As a result, the plasma jet formed by the tungsten electrode and the anode can be produced such that it is thermally stable. The cooling device may preferably be water cooling.
It is also preferable if an electrical voltage ranging from 5 V to 400 V is applied between the tungsten electrode and the anode to generate the plasma. The electrical voltage may in particular range from 5 V to 300 V, particularly from 5 V to 100 V.
It is also preferable if an electric current ranging from 10 kA to 300 kA flows between the tungsten electrode and the anode to generate the plasma.
When generating the plasma to clean the joining surface, a stable arc can be produced with relatively low voltages and relatively high currents.
The diameter of the plasma gas nozzle preferably ranges from 1 mm to 10 mm.
According to the present invention, the joining method further comprises the step of generating an ignition tip on the joining surface.
Further according to the present invention, the plasma used in the cleaning process is also used to generate the ignition tip. Thus, the same plasma is used for the cleaning and to provide an ignition tip. The ignition tip may be generated after or during the cleaning process. The same device cleans therefore the surface and generates an ignition tip.
In an embodiment the plasma is used to locally melt the joining surface and forms the ignition tip.
In an embodiment, the ignition tip comprises a circular cross-section. More specifically, the plasma (or plasma jet) creates a circular projection which projects from a general flat plane of the joining surface. This projection forms the ignition tip.
In an embodiment, the ignition tip is provided on the second joining surface.
In an embodiment, the joining element is joined to the component through arc welding, with drawn-arc ignition, and wherein the joining process comprises:

placing the first joining surface adjacent the ignition tip of the second joining surface and switching on an electric pilot current,
lifting the joining element away from the component,
flowing a welding current through the arc in such a manner that the first joining surface and second joining surface start to melt,
lowering the joining element onto the component, wherein the melts of the first and second joining surfaces mix,
switching off the welding current so that the entire melt solidifies to join the joining element and the component.

It is assumed that the above-mentioned features and the features still to be explained below can not only be used in the respective specified combination, but also in other combinations or in isolation, without deviating from the scope of the present invention.
Embodiments of the invention are shown in the drawings and explained in greater detail in the following description. These drawings are as follows:

Fig. 1 is a schematic representation of a joining device according to an embodiment of the invention;
Fig. 2 is a schematic representation of a plasma gas cleaning device;
Fig. 3 is a schematic representation of a snow jet cleaning device;
Fig. 4 is a schematic representation of a TIG arc cleaning device;
Fig. 5 is a schematic plan view of a joining surface;
Fig. 6 is a schematic representation of another embodiment of a joining device according to the invention from the side;
Fig. 7 shows the joining device in Fig. 6 from the front.
Fig. 8a to 8e show different steps of a joining method with a plasma cleaning method and the generation of an ignition point on the joining surface.

Fig. 1 is a schematic representation of a joining device for joining joining elements to components, generally referred to as 10.
The joining device 10 comprises a joining head 12, which can be moved freely in the space by means of a robot 14, said joining head 12 preferably being mounted on one arm 16 of the robot 14 in this case.
A carriage 18 can preferably be moved along a joining axis 20 on the joining head 12. The maximum stroke of the carriage 18 is preferably larger than a maximum joining stroke.
A retaining device 22 to retain a joining element 24 is arranged on the carriage 18. The joining element 24 may, for example, be designed as a stud, with a shaft portion which is not shown in greater detail, and a flange portion which is not shown in greater detail, a first joining surface 26 being formed on one side of the flange portion facing away from the shaft portion. The joining element 24 is preferably made from aluminum or aluminum alloy.
The joining element 24 can be joined to a component 28 such as a plate by means of the joining device 10, the component 28 preferably also being made from aluminum or an aluminum alloy.
A second joining surface 30 is formed on the component 28, said surface having a diameter D_FB, which approximately corresponds to the diameter of the flange portion of the joining element 24.
A coating 32 may be formed on the joining surface 30, said coating being formed of release agents or waxes, oils, polysiloxanes, hydrocarbons, polymers, etc.
The joining device 10 is in particular designed as a stud welding device, but may also be in the form of a stud bonding/ stud gluing device.
The joining device 10 comprises a cleaning device 34, by means of which the second joining surface 30 can be cleaned before carrying out the joining process. The cleaning device 34 is preferably designed to direct a cleaning medium onto the second joining surface 30, and specifically along a longitudinal axis 36, which is oriented at an angle a with respect to the second joining surface 30. The angle α may, for example, range from 30° to 90°, and particularly from 30° to 85°.
In an embodiment (not shown in the figures), the first joining surface can be cleaned before carrying out the joining process by the joining device 10. In another embodiment, the first and second joining surfaces might be cleaned simultaneously and/or both surfaces might be cleaned by the cleaning device 34.
As illustrated, the cleaning device 34 is attached to the joining head 12, but may also be designed to be independent from the joining head 12.
Furthermore, the joining device 10 may comprise a recording device 38, which is able to record the status of the second joining surface 30 and/or a surface coating on the second joining surface 30. In particular, the recording device 38 is designed to record a characteristic variable of the component 28.
In this case the cleaning device 38 is attached to the joining head 12, but may also be designed to be independent from said joining head 12.
In order to provide high quality joints, and especially to provide consistent joints, it is preferable for each joining surface 30 to be first processed by the recording device 38 before carrying out a joining process on said surface, after which the characteristic variable thus recorded is evaluated. A decision can be made on the basis of this variable whether a joining process can be performed immediately afterwards, or whether it is desirable or necessary to perform a cleaning process using the cleaning device 34 beforehand.
Figure 2 shows a cleaning device 34-1 in the form of a plasma gas cleaning device.
The plasma gas cleaning device 34-1 comprises an elongated tungsten electrode 40, which preferably extends coaxially in relation to a joining axis 20 or cleaning axis 20.
The cleaning device 34-1 also comprises an anode sleeve 42, an annular intermediate space 44 being formed between the tungsten electrode 40 and the anode sleeve 42.
A plasma gas 45 is admitted to the intermediate space 44. An arc voltage U is applied between the tungsten electrode 40 and the anode sleeve 42, causing a corresponding current I to flow.
Plasma 49 is generated between the tungsten electrode 40 and the anode sleeve 42 from the plasma gas 45 as a result of this arc voltage U and the current I, said plasma emerging from a plasma gas nozzle 46 arranged at one downstream end of the anode sleeve 42.
As a result, a kind of plasma arc (or plasma jet) is generated from the plasma gas nozzle 46 towards the second joining surface 30, this arc being a non-transmitted arc (or non-transferable arc), and preferably not undergoing any magnetic deflection due to ground effects.
The space A between the plasma gas nozzle 46 and the second joining surface 30 may, for example, range from 2 mm to 25 mm. The internal diameter Do of the plasma gas nozzle may, for example, range from 2 mm to 15 mm.
Fig. 2 also shows that the arrangement of the tungsten electrode 40 and the anode sleeve 42 may be cooled by a cooling device 50, for example by water cooling. As a result, this arrangement can be made more thermally stable.
As a general rule, it is not necessary to supply an inert gas around the plasma arc 48, as is known from TIG welding, for example. If this is still necessary for specific reasons, an inert gas sleeve 52 may be arranged around the outside of the anode sleeve 42 such that an inert gas 54 can be supplied between the inert gas sleeve 52 and the anode sleeve 42.
Fig. 3 shows a snow jet cleaning device 34-2 in which a gas 60 such as CO₂ and compressed air are passed into a snow jet nozzle 64 from a compressed air generator 62. In this process the gas 60 is first compressed and then expanded in the snow jet nozzle such as to produce snow or ice crystals 66 in the snow jet nozzle 64.
The internal diameter D_D' of the snow jet nozzle may, for example, range from 1 mm to 5 mm.
The snow crystals 66 carried by the compressed air flow impact on and break up a coating 32, as illustrated schematically in Fig. 3.
In the snow jet cleaning device 34-2, it may be preferable if a joining or cleaning axis 20 is oriented at an angle a in relation to the joining surface 30, said angle ranging from 30° to 85°.
Fig. 4 shows a TIG arc cleaning device 34-3. In this case, an arc voltage is applied between a tungsten electrode 40' and the component 28 such that a TIG arc 17 is created between the tungsten electrode 40' and the component 28 in the region of the joining surface 30. If applicable, an inert gas sleeve 52' may be provided around the tungsten electrode 40' such that the TIG arc 70 can be surrounded by an inert gas 54.
Fig. 5 shows a plan view of a joining surface 30 of a component 28, said joining surface having a diameter D_FB.
A radius of the joining surface 30 is shown as r.
Various positions on a plasma arc 48 (or a snow jet) directed onto the joining surface 30 are shown as 48.
It is evident that the diameter D_R of this plasma arc 48 (or the snow jet) may be greater than or equal to the diameter D_FB, but may also be smaller. An effective overall cleaning surface can be achieved by moving the plasma arc 48 (or the snow jet) in relation to the second joining surface 30, for example on a circular path 74. It is also possible to position the plasma arc 48 (or the snow jet) at an angle in relation to the joining surface 30 such as to produce an overall tumbling motion.
Figs. 6 and 7 show another embodiment of a joining device 10' which generally corresponds to the joining device 10 shown in Fig. 1 with regard to its structure and mode of operation. The same components are therefore identified by the same reference numerals.
The joining device 10' comprises a motor 80, which is fixed to the joining head 12, a cleaning device 34 being able to rotate around an axis of rotation, which is oriented transversely with respect to the joining axis 20. In this case the motor 80 is connected to the cleaning device 34 via an interface 82. The direction of rotation 84 around the axis of rotation is shown in Fig. 7. A displacement measurement device 86 is preferably assigned to the cleaning device 34 and used to record the angle of rotation.
The angle a at which a cleaning medium is directed onto a joining surface 30 of the component 28 can be adjusted by means of the motor 80 as a result.
Fig. 8a to Fig. 8e show different steps of a joining method according to the invention. The cleaning device 34 is according to the present invention a cleaning device 34-1 in the form of a plasma gas cleaning device.
As illustrated in Fig. 8a and Fig. 8b, the plasma 49 or a plasma jet is used to clean the joining surface 26, 30, and in particular the second joining surface 30 as described above. The plasma 49 or plasma jet will first clean the joining surface (in particular the second joining surface 30). Any lubricant or contamination provided on the joining surface are removed through the plasma 49 or plasma jet. The plasma jet is in particular generated by a power source. Through the thermal effect of the plasma, the coating 32 (which can be as previously mentioned oils, polymers, contaminations ...) is vaporized, burnt and/or removed.
The plasma 49 or plasma jet is further applied in order to create a local melting of the joining surface, as shown in Fig. 8c. The parameters used to generate the plasma during the cleaning step might be modified to provide the melting area. The pressure applied by the plasma on the melting area generates a projection or ignition tip 56. The projection or ignition tip 56 has a circular shape or a circular cross section. For example, the projection or ignition tip 56 has a crater-like shape.
The ignition tip 56 enables a better welding of the joining element on the component, as already known from the prior art. The generation of the joining tip 56 on the component 28 and not on the joining element 24, allows to avoid a preforming of the joining element 24. Thus, the shape of the joining element 24 might be randomly chosen and its end face (or joining surface) may not need to be prepared.
More particularly, after forming the ignition tip 56, the joining element 24 may be joined to the component 28 through arc welding, with drawn-arc ignition. In a first step, the first joining surface 26 is placed adjacent the ignition tip of the second joining surface 30. An electric pilot current is switched on. The joining element 24 is then lifted away from the component 28 with the retaining device 22. The welding current flows through the arc in such a manner that the first joining surface 26 and second joining surface 30 start to melt. More particularly, the second joining surface starts to melts from the ignition tip, which allows a better repartition of the melting. The ignition tip 56 allows the arc to remain in a precise location.
The joining element 24 is then lowered onto the component 28, and the melts of the first and second joining surfaces 26, 30 mix. The welding current is switched off and the entire melt solidifies to join the joining element 24 and the component 28, as visible in Fig. 8e. The retaining device 22 can then be moved away from the assembly, for example by following the direction of the arrow shown in Fig. 8e.

Claims

Method of joining by welding joining elements (24) to components (28) with the following steps:
- providing a joining element (24), which comprises a first joining surface (26), and providing of a component (28), which comprises a second joining surface (30);

- preparing the first and/or second joining surface (26; 30); and

- carrying out a joining process, in which the joining element (24) is joined to the component (28);
characterized in that the preparing step comprises at least one plasma gas cleaning method, and wherein the plasma gas cleaning method includes generating a non-transferable arc (48) between a tungsten electrode (40) and an anode (42) surrounding the tungsten electrode (40), wherein the non-transferable arc (48) generates a plasma (49) when using a plasma gas (45), and wherein said plasma (49) is directed onto the joining surface (26), and in that the joining method further comprises the step of generating an ignition tip (56) on the joining surface (26; 30), wherein the plasma (49) used in the cleaning process is also used to generate the ignition tip, the plasma (49) locally melting the joining surface (26;30) to form the ignition tip.
Joining method according to claim 1, wherein the plasma gas (45) is conducted under pressure into an intermediate space (44) between the tungsten electrode (40) and the anode (42), and wherein the plasma (49) is discharged from the intermediate space (44) towards the joining surface (26).
Joining method according to claim 2, wherein the anode (42) is connected to a plasma gas nozzle (46) located downstream in the direction of the plasma gas discharge direction, wherein said nozzle bundles the plasma gas (49) emerging from the intermediate space (44).
Joining method according to claim 3, wherein a distance ranging from 2 mm to 25 mm is adjusted between the plasma gas nozzle (46) and the joining surface (26; 30) during the cleaning step and/or wherein a ratio between a nozzle diameter (Do) of the plasma gas nozzle (46) and a distance (A) between the plasma gas nozzle (46) and the joining surface (26; 30) during the cleaning step ranges from 1:4 to 1:1 and/or wherein the anode (42) and/or the plasma gas nozzle (46) connected to the anode (42) is cooled by means of a cooling device.
Joining method according to any one of claims 1 to 4, wherein an electric voltage (U) ranging from 5 Volts to 400 Volts is applied between the tungsten electrode (40) and the anode (42) to generate the plasma (49) and/or an electric current (I) ranging from 10 kilo-amperes to 300 kilo-amperes flows between the tungsten electrode (40) and the anode (42) to generate the plasma (49).
Joining method according to claim 6 or claim 7, wherein the ignition tip (56) is provided on the second joining surface (30).
Joining method according to any one of claims 6 to 8, wherein the joining element (24) is joined to the component (28) through arc welding, with drawn-arc ignition, and wherein the joining process comprises:
- placing the first joining surface (26) adjacent the ignition tip of the second joining surface and switching on an electric pilot current,

- lifting the joining element (24) away from the component (28),

- flowing a welding current through the arc in such a manner that the first joining surface (26) and second joining surface (30) start to melt,

- lowering the joining element (24) onto the component (28), wherein the melts of the first and second joining surfaces (26, 30) mix,

- switching off the welding current so that the entire melt solidifies to join the joining element (24) and the component (28).